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United States Patent |
5,696,595
|
Yamanishi
|
December 9, 1997
|
Image forming apparatus with automatic density adjustment
Abstract
A histogram preparation circuit prepares a density histogram from image
data sent from a scanner. A histogram clear discriminating section detects
a white peak of the histogram and a black peak thereof, and compares
density of the white peak and that of the black peak with a white side
threshold value and a black side threshold value, respectively. When the
density of the white peak is the white side rather than the white side
threshold value or the density of the black peak is the black side rather
than the black side threshold value, it is detected whether or not a sum
of the frequencies around the density of the white or black peak is a
predetermined value or more. When the sum of the frequencies is the
predetermined value or more, a histogram value stored in a histogram
preparation circuit is cleared. A histogram is prepared by use of only
image data showing the document, so that a suitable automatic density
adjustment can be executed.
Inventors:
|
Yamanishi; Eiichi (Yokohama, JP)
|
Assignee:
|
Kabushiki Kaisha Toshiba (Kawasaki, JP)
|
Appl. No.:
|
570552 |
Filed:
|
December 11, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
358/3.23; 382/168 |
Intern'l Class: |
G01N 021/55; G03G 013/06 |
Field of Search: |
358/298,455.6,465.6,475.6
382/168-172
399/46
|
References Cited
U.S. Patent Documents
5075788 | Dec., 1991 | Funada | 358/458.
|
5351313 | Sep., 1994 | Bessho et al. | 382/51.
|
5383032 | Jan., 1995 | Euguchi et al. | 358/448.
|
5467196 | Nov., 1995 | Fukushima et al. | 358/298.
|
5471319 | Nov., 1995 | Ogawa | 358/445.
|
5502776 | Mar., 1996 | Manabe | 382/172.
|
5585927 | Dec., 1996 | Fukui et al. | 358/298.
|
Foreign Patent Documents |
3-030143 | Apr., 1991 | JP.
| |
2086077 | Aug., 1981 | GB | 358/298.
|
Primary Examiner: Wong; Peter S.
Assistant Examiner: Toatley, Jr.; Gregory J.
Attorney, Agent or Firm: Foley & Lardner
Claims
What is claimed is:
1. An image forming apparatus comprising:
a document plate on which a document is placed, the document plate having a
plurality of reading areas;
means for reading an image in the reading area of said document plate as
moving along a scanning direction so as to output a density signal for
each pixel;
means for preparing a first density distribution corresponding to each
reading area based on the density signal;
means for discriminating that the reading area corresponds to the document
from the first density distribution;
first calculating means for accumulatively calculating data of the first
density distribution in accordance with the movement of said reading means
along said scanning direction so as to prepare a second density
distribution;
second calculating means for calculating a density correction reference
value of each of said reading areas based on the first density
distribution of each of said reading areas when said discriminating means
discriminates that said reading area does not correspond to the document,
and for calculating a density correction reference value of each of said
reading areas based on the second density distribution without
accumulating the first density distribution data of the reading area
discriminated as said reading area where said reading means does not read
the document when said discriminating means discriminates that said
reading means reads the document;
means for correcting the density signal by using the density correction
reference value so as to output a corrected density signal; and
means for forming an image on a image bearing member on the basis of the
corrected density signal.
2. The apparatus according to claim 1, wherein said discriminating means
includes means for detecting a frequency peak of the density distribution
of each of the predetermined areas prepared by said first preparing means
and for discriminating whether or not density of said frequency peak is a
black side rather than a black side threshold value, and said correction
reference value calculating means includes means for clearing density
distribution data prepared by said first preparing means when density of
said frequency peak is the black side rather than the black side threshold
value.
3. The apparatus according to claim 1, wherein said discriminating means
includes means for detecting a frequency peak of the density distribution
of each of the predetermined areas prepared by said first preparing means
so as to discriminate whether or not density of said frequency peak is a
black side rather than a black side threshold value, and said correction
reference value calculating means includes means for summing the
frequencies around said density of said frequency peak when density of
said frequency peak is the black side rather than said black side
threshold value so as to clear the density distribution prepared by first
preparing means when the summed value is larger than the predetermined
value.
4. The apparatus according to claim 3, wherein said correction reference
value calculating means includes means for summing the frequencies for a
density width containing the density of said frequency peak when the
density of said frequency peak is the black side rather than said black
side threshold value so as to clean the density distribution prepared by
first preparing means when the summed value is larger than the
predetermined value.
5. The apparatus according to claim 3, wherein said discriminating means
includes means for detecting a frequency peak of the density distribution
of each of the predetermined areas prepared by said first preparing means
so as to discriminate whether or not the density of said frequency peak is
a black side rather than a black side threshold value and said frequency
peak is a white side rather than a white side threshold value, and said
correction reference value calculating means includes means for summing
the frequencies around density of higher frequency peak when density of
said frequency peak is the black side rather than said black side
threshold value and is the white the side rather than said white side
threshold so as to clear the density distribution prepared by first
preparing means when the summed value is larger than the predetermined
value.
6. An image forming apparatus comprising:
a document plate on which a document is placed, the document plate having a
plurality of reading areas;
means for reading an image in the reading area in a main scanning direction
as moving on said reading area of said document plate along a sub-scanning
direction so as to output a pixel density signal every scanning line;
means for preparing a 16 level density histogram for each scanning line
from the density signal output from said reading means;
means for discriminating that the scanning line is on the document from
said histogram;
means for clearing said histogram when said discriminating means
discriminates that said scanning line is not on the document;
first calculating means for accumulating the histograms by use of a
weighting coefficient changing in accordance with number of scanning line
counts of said sub-scanning direction when said reading means is moved
along said sub-scanning direction, whereby preparing histograms
accumulated up to a present line;
second calculating means for calculating a density correction reference
value for each of said scanning lines based on the histogram accumulated
by said first calculating means;
means for correcting the density signal by using the density correction
reference value so as to output a corrected density signal; and
means for forming an image on a image bearing member on the basis of the
density signal.
7. The apparatus according to claim 6, wherein said discriminating means
includes means for detecting a frequency peak of the histogram of each of
said scanning lines prepared by said first preparing means so as to
discriminate whether or not density of said frequency peak is a black side
rather than a black side threshold value, and said clearing means includes
means for clearing the histogram prepared by said first preparing means
when density of said frequency peak is the black side rather than the
black side threshold value.
8. The apparatus according to claim 6, wherein said discriminating means
includes means for detecting a frequency peak of the histogram of each of
said scanning lines prepared by said first preparing means so as to
discriminate whether or not density of said frequency peak is a black side
rather than a black side threshold value, and said clearing means includes
means for summing the frequencies around density of said frequency peak
when density of said frequency peak is the black side rather than said
black side threshold value so as to clear the histogram prepared by first
preparing means when the summed value is larger than a predetermined
value.
9. The apparatus according to claim 8, wherein said clearing means includes
means for summing the frequencies for a density width centrally containing
the density of said frequency peak when the density of said frequency peak
is the black side rather than said black side threshold value so as to
clean the density distribution prepared by first preparing means when the
summed value is larger than the predetermined value.
10. The apparatus according to claim 6, wherein said discriminating means
includes means for detecting a frequency peak of the histogram of each of
said scanning lines prepared by said first preparing means so as to
discriminate whether or not a density of said frequency peak is a black
side rather than a black side threshold value and said density is a white
side rather than a wide side threshold value, and clearing means includes
means for summing the frequencies around density of higher frequency peak
when density of said frequency peak is the black side rather than said
black side threshold value and is the white the side rather than said
white side threshold so as to clear the histogram prepared by first
preparing means when the summed value is larger than the predetermined
value.
11. A method for forming an image, comprising the steps of:
reading an image of a reading area of a document plate to output a density
signal of each pixel, the document plate having a plurality of reading
areas;
preparing a first density distribution corresponding to each reading area
based on the density signal;
discriminating that the reading area corresponds to the document from the
density distribution;
clearing the density distribution when said reading area does not
correspond to the document;
accumulatively calculating said density distribution to prepare an
accumulative density distribution;
calculating a density correction reference value of each of said reading
areas based on said accumulative density distribution;
correcting said density signal obtained in said image reading step based on
said density correction reference value so as to output a corrected
density signal; and
forming an image on a image bearing member on the basis of the corrected
density signal.
12. An image forming apparatus comprising:
a document plate on which a document is placed, the document plate having a
plurality of a reading areas;
means for scanning an image of the reading area of said document plate so
as to output a density signal for each pixel;
means for preparing a density distribution based on the density signal;
means for calculating a density correction reference value of the density
signal based on the density distribution prepared by said preparing means;
means for correcting the density signal by using the density correction
reference value so as to output a corrected density signal;
means for forming an image on an image bearing member on the basis of the
corrected density signal; and
means for preventing an abnormal value from being calculated in the
operation of the reference value calculation by said calculating means
when the reading area corresponds to the area where no document is placed.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as a
digital copy machine for obtaining a suitable image using a histogram
based on density.
2. Description of the Related Art
In recent years, a digital image forming apparatus such as an electronic
copy machine has been widely used in addition to the conventional analog
image forming apparatus. Under such a circumstance, there has been tried
to realize an automatic exposing function, which is the general function
in the analog copy machine, in the digital copy machine. More
specifically, the automatic exposing function is the function in which a
density of a document is detected by a sensor and illumination of an
exposure lamp is varied so as to obtain an image having a suitable
quality. There is proposed a digital copy machine in which an automatic
density adjustment is performed by use of a density histogram, as in
Japanese Patent Application KOKOKU Publication Nos. 64-6588 and 3-30143.
In the automatic density adjustment of the digital copy machine using the
histogram, since a reference value for correcting the density of the image
from a numeral value of the histogram, a suitable reference value for
correction can not be calculated unless the histogram correctly shows a
density distribution on the document. Therefore, there occurs a problem in
which the automatic density adjustment cannot be performed, or a defective
image is outputted.
For example, as shown in FIG. 18, in a case where the document is placed on
a document glass 92 without being come into contact with a document scale
91, and a document cover is opened, a portion (x portion) other than the
document is read as an entire black portion. Due to this, black portion
data is stored on the histogram, and a suitable reference value for
correction cannot be obtained.
As an another example, in a case where the document is not placed at an
appropriate position even if the document cover closed, a white portion of
the document cover is accumulated on the histogram. Therefore, in a case
where paper having a base color like a newspaper is used as a document,
the automatic density adjustment cannot be suitably performed.
As mentioned above, in the conventional image forming apparatus such as a
digital copy machine which performs the automatic density adjustment by
use of the histogram, the entire black portion other than the document or
the white portion on the document cover are reflected on the accumulative
histogram. Due to this, there was a problem in which a prepared histogram
does not correctly show the density distribution on the document and a
suitable automatic density adjustment cannot be performed.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an image forming apparatus
in which a histogram on a portion other than a document is canceled to
prepare a suitable histogram, and an automatic density adjustment can be
performed by use of a suitable correction reference value.
In order to achieve the above object, according to the present invention,
there is provided an image forming apparatus comprising: a document plate
for mounting a document thereon; reading means for reading an image of a
reading area of the document plate every predetermined area as moving
along a scanning direction so as to output a density signal for each
pixel; first preparing means for preparing a density distribution of each
of the predetermined areas from the density signal output from the reading
means; discriminating means for discriminating whether or not the
predetermined area corresponds to the document from the density
distribution prepared by the first preparing means; second preparing means
for accumulatively calculating data of the density distribution prepared
by the first preparing means in accordance with the movement of the
reading means along the scanning direction so as to prepare a density
distribution of data; correction reference value calculating means for
calculating a density correction reference value of each of the
predetermined areas based on the density distribution of each of the
predetermined areas prepared by the first preparing means when the
discriminating means discriminates that the predetermined area does not
correspond to the document, and for calculating a density correction
reference value of each of the predetermined areas based on the density
distribution of each of the predetermined areas prepared by the second
preparing means without accumulating density distribution data of the
first preparing means of an area discriminated as an area where the
reading means does not read the document when the discriminating means
discriminates that the reading means reads the document; and image forming
means for correcting the density signal output from the reading means
based on the density correction reference value calculated by the
correction reference value calculating means so as to form an image by use
of the corrected density signal.
The above discriminating means detects a frequency peak of the density
distribution of each of the predetermined areas prepared by the first
preparing means so as to discriminate whether the density of the frequency
peak is a black side rather than a black side threshold value or a white
side rather than a white side threshold value. The above clearing means
sums the frequencies around the density of the frequency peak when the
density of the frequency peak is the black side rather than the black side
threshold value or the density of the frequency peak is the white side
rather than the white side threshold value so as to clean the density
distribution prepared by the first preparing means when the summed value
is larger than a predetermined value.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
invention and, together with the general description given above and the
detailed description of the preferred embodiments given below, serve to
explain the principles of the invention.
FIG. 1 is a cross sectional view showing a schematic structure of an image
forming apparatus of the present invention;
FIG. 2 is a block diagram showing a schematic structure of a controlling
system of the image forming apparatus of the present invention;
FIG. 3 is a block diagram showing a schematic structure of an image
processing section of the image forming apparatus of the present
invention;
FIGS. 4A and 4B are views explaining a histogram prepared in the present
invention;
FIGS. 5A and 5B are histogram views explaining a correction reference value
and a range correction;
FIG. 6 is a view showing a number of sub-scanning lines and their
corresponding coefficient .alpha.;
FIG. 7 is a flow chart showing an operation of a histogram preparing
circuit;
FIG. 8 is a block diagram showing a structure of the histogram preparing
circuit in the image forming apparatus of one embodiment of the present
invention;
FIG. 9 is a view showing a change of each signal corresponding to a change
of a signal FDAT of the histogram preparing circuit of FIG. 8;
FIG. 10 shows a relationship between input and output signals of a clock
generating section 64 of the histogram preparing circuit of FIG. 8;
FIG. 11 is a view showing an example of an output of an addition value
generating section;
FIG. 12 is a view showing a change of each signal corresponding to a change
of a signal FDAT of the histogram preparing circuit of FIG. 8;
FIG. 13 is a view showing an example of an addition of signals ZDAT of the
histogram preparing circuit of FIG. 8;
FIG. 14 is a timing chart explaining an operation of the histogram
preparing circuit of FIG. 8;
FIG. 15 is a timing chart explaining an operation of the histogram
preparing circuit of FIG. 8;
FIG. 16 is a flow chart explaining image data processing based on the
histogram according to the present invention;
FIGS. 17A to 17C are views each showing an example of a histogram by a
scanning sampling on one line; and
FIG. 18 is a view explaining a state where a document is placed on a
document glass.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be explained with reference to
the drawings.
FIG. 1 shows a schematic structure of an image forming apparatus to which
the present invention is applied. The image forming apparatus comprises a
scanner section 1 for reading a document, and a printer 2 for forming an
image on paper in accordance with an image signal supplied from the
scanner section 1 or an outer unit (not shown).
The scanner section 1 comprises a document plate 117 on which a document to
be copied is mounted, an openable/closable document cover 109 for pressing
the document mounted on the document plate 117, a fluorescent lamp 3,
serving as a light source for emitting light to the document mounted on
the document plate 117, and a CCD typed line sensor 4 for
photoelectrically converting a reflected light from the document on which
light is emitted by the fluorescent lamp 3. The fluorescent lamp 3 has a
lamp heater (not shown) serving as heating means for heating a tube wall
at a fixed temperature. The document plate 117 has a document scale 91
which is used such that the document is run against a document glass 92 on
which the document is mounted so as to measure the position of the
document.
A reflector 115 is provided on the side portion of the fluorescent lamp 3.
The reflector 115 is used to efficiently converge light sent from the
fluorescent lamp 3 on the document. A plurality of mirrors 112 to 114 and
a lens unit 116 are provided between the fluorescent lamp 3 and the line
sensor 4. The mirrors 112 to 114 are used to bend an optical path through
which light directing to the line sensor 4 from the document, that is, the
reflected light from the document. The lens unit 116 is used to focus the
reflected light on a light receiving surface of the line sensor 4.
A scanning system comprising the fluorescent lamp 3 and the mirrors 112 to
114 are moved back and forth in a direction of an arrow a along a lower
surface of the document plate 117. The document mounted on the document
plate 117 is exposed and scanned at the time when the scanning system is
moved back. In this case, the mirrors 113 and 114 are moved at a half
speed of the mirror 112 to maintain a length of the optical path.
The reflected light, which is sent from the document by the scanning of the
above scanning system, that is, the reflected light from the document on
which light emitted by the fluorescent lamp 3, is reflected by the mirrors
112 to 114. Thereafter, the reflected light is passed through the lens
unit 116 to be guided to the line sensor 4, so that an image of the
document is image-formed on the receiving surface of the line sensor 4.
A scanning unit 108 comprises the fluorescent lamp 3, the line sensor 4,
the mirrors 112 to 114, and the lens unit 116. The fluorescent lamp 3, the
reflector 115, and the mirror 112 are provided at a first carriage 111,
and the mirrors 113 and 114 are provided at a second carriage 110. These
carriages 111 and 110 are moved by a motor (not shown), respectively.
The printer section 2 has a photosensitive drum 6, which is cylindrically
shaped, and serves as an image carrier member. The photosensitive drum 6
is structured to be rotatable in a predetermined direction by a motor (not
shown). The drum 6 is charged to a predetermined voltage. Also, the drum 6
is irradiated with a light beam, which is modulated in accordance with
print data, thereby an electrostatic image is formed thereon.
A charging device 102, a laser unit 5, a developer 7, a transferring device
105, and a separating device 106 are provided around the photosensitive
drum 6. The charging device 102 charges the surface of the photosensitive
drum 6. The laser unit 5 outputs a laser beam, which is modulated in
accordance with print data serving as image data to be copied or outputted
onto the surface of the photosensitive drum 6. The developer 7 develops
the electrostatic image, which is formed on the photosensitive drum 6 with
the light beam from the laser unit 5, by adhering toner to the
electrostatic image. The transferring device 105 transfers a developed
toner image, which is formed on the photosensitive drum 6, to paper
supplied from a paper supply section 9 to be described later. The
separating device 106 separates paper absorbed on the photosensitive drum
6.
A cleaner unit 104 and an erasing device 107 are provided in order at a
portion, which is the surroundings of the photosensitive drum 6 and a
lower side of the separating device 106. The cleaner unit 104 cleans toner
left on the surface of the photosensitive drum 6. The erasing device 107
erases the potential charged on the photosensitive drum 6 in order to form
a next image.
The paper supply section 9 is provided between the developing device 7 and
the transferring device 5. Paper, to which the toner image formed on the
photosensitive drum 6 is transferred, is supplied to a portion between the
photosensitive drum 6 and the transferring device 105 by the paper supply
section 9.
In a direction where paper to which the toner image is transferred is
separated from the drum 6 by the separating device 106, a fixing device 8
and a delivering device 103 are provided. The fixing device 8 fixes the
toner image onto paper. The delivering device 103 delivers paper separated
by the separating device 106 to the fixing device 8. Paper to which the
toner image is fixed by the fixing device 8 is discharged to a discharging
tray 10 by a discharging roller 119.
FIG. 2 is a block diagram showing a schematic structure of the control
system of the image forming apparatus. The apparatus is controlled by a
main CPU 11, a control panel CPU 12, a scanner CPU 13, and a printer CPU
14. The main CPU 11 communicates with the control panel CPU 12, the
scanner CPU 13 and the printer CPU 14 to control these CPUs.
The control panel CPU 12 is connected to a ROM 15 and a RAM 16. The control
panel CPU 12 detects a switch formed on the control panel 17, turns on/off
an LED, and controls a display based on data stored in the ROM 15 and the
RAM 16. The scanner CPU 13 is controlled by the communication with the
main CPU 11. The scanner CPU 13 controls mechanical components 23 such as
a motor (not shown), and a solenoid, etc., an ADF (automatic document
feeder) 24, an editor 25, an A/D (analog to digital) converter 26, an SHD
(shading correcting circuit) 27, and a line memory 28.
The printer CPU 14 is controlled by the communication 2 with the main. CPU
11. The printer CPU 14 controls mechanical components 33 such as a motor
(not shown), a solenoid, etc., a sorter 34, an LCF (large cassette feeder)
35, a laser modulation circuit 36, and a laser drive circuit 37 based on
data stored in a ROM 31 and a RAM 32.
The main CPU 11 totally controls the image forming apparatus in accordance
with a control program stored in a ROM 41 and a RAM 42. A data change and
buffer memory 43 changes and buffers where data read by the scanner
section 1 should be sent and which data should be sent to the printer
section 2. In an image processing section 44, there are provided a circuit
for preparing a histogram from image data so as to correct image data
based on the prepared histogram, and an automatic density adjusting
section of the present invention. A compressing and expanding circuit 45
performs compression and expansion of image data, and a page memory
circuit 46 stores image data every page. A display memory 48 stores image
data to be displayed on a display 47. A printer controller 50 expands code
data sent from a personal computer 49 to image data. A display font ROM 51
expands code data onto the display memory 48, a print font ROM 52 expands
code data onto the page memory 46, a compression memory 53 stores data
compressed by the compressing and expanding circuit 45. In addition to the
above-explained components, a hard disk drive 54, an optical disk drive
55, and an I/F controller 57, which carries out an interface with a
facsimile adapter 56, are connected to the main CPU 11.
FIG. 3 is a block diagram showing a schematic structure of the image
processing section 44. A histogram preparation circuit 80 prepares a
density histogram from image data sent from the scanner section 1. A
histogram clear discriminating section 88, serving as discriminating
means, discriminates whether or not the histogram prepared by the
histogram preparation circuit 80 should be cleared based on a white side
threshold value and a black side threshold value (to be described later),
which are set by the main CPU 11.
A correction reference value calculation section 81 calculates a correction
reference value (to be described later) based on the histogram prepared by
the histogram preparation circuit 80. A range correction circuit 82
corrects a density range (to be described later) by use of the correction
reference value from the correction reference calculation section 81 to
perform the automatic density adjustment at real time.
A timing signal generating section 83 generates various timing signals
necessary for each block of the image processing section 44 based on a
clock signal from a clock generating section 84. An image improving
circuit 85 includes a low pass filter, and a high frequency emphasizing
circuit to improve the quality of the image range-corrected by the range
correction circuit 82. An expansion/reduction circuit 86 expands/reduces
an image as required. A gradation processing circuit 87 processes a
gradation of an image by a dither method or an error diffusion method. The
above processed image signal is sent to the printer section 2, and an
image is formed.
FIG. 4 shows an outline of a density histogram prepared by the histogram
preparation circuit 80. For example, for reading an image of one A4 paper,
if the image is read at 400 dpi, the total number of pixels G is shown as
follows:
G=210.times.297.times.(400/25.4).sup.2
Each pixel has its density, and the density is expressed by 8 bits herein.
In FIG. 4A, a horizontal axis shows a density, that is, a pixel value, and
a vertical axis is a frequency (number of pixels) showing how many pixels
exist at each density of pixel.
As shown in FIG. 4A, according to this embodiment, the density is divided
to sixteen, and the density having 256 levels is simplified to 16 levels.
In other words, lower 4 bits of 8-bit pixel value are ignored. By use of
16 divisions, the hardware can be largely simplified. Even in the case of
16 divisions, an amount of data necessary for histogram can be fully
reversed in the automatic density adjusting function. FIG. 4B shows a
method of 16 even divisions, a division number 0 shows a range of the
pixel density of 0 to F, a division number 1 shows a range of the pixel
density of 10 to 1 F. Similarly, the pixel value ranges of division
numbers 2 to F are set.
The following will explain the correction reference value calculation
section 81 and the range correction of the range correction circuit 82.
The range correction is a function, which is used in a background deletion
in the automatic exposure function of the analog copy machine.
Generally, if the document is digitally read and a density histogram is
prepared, the result can be obtained as shown in FIG. 5A. In a case of the
document like newspaper, one curve whose peak is M can be formed at a
background density portion and one curve whose peak is N can be formed at
a character density portion. In the analog copy machine, an exposure lamp
is controlled such that the background density portion can be excluded.
However, since the background density portion cannot be excluded in the
digital copy machine, the similar effect can be obtained by the following
method.
More specifically, density DW corresponding to the peak point of frequency
M, and density DB corresponding to the peak point of frequency N are
obtained, and the following calculation is performed, so that a density
histogram is converted to a distribution as shown in FIG. 5B. The
densities DW and DB are called as correction reference values, and each
correction reference value is calculated by the correction reference value
calculation section 81 based on the histogram of each scanning line which
is prepared by the histogram preparation circuit 80.
DN=(DI-DW).times.FFH/(DB-DW)
where DI is an input pixel density, DN is a corrected pixel density, and
FFH is a maximum pixel density. In other words, the range (density width)
between M to N shown in FIG. 5A is expanded from 0 to FFh.
The following will explain the histogram preparation system of system of
the present invention.
The following equation is a basic calculation expression for preparing the
histogram of the present invention. The histogram is prepared every main
scanning line. A basic reference value of the range correction is obtained
every time when the histogram preparation processing is executed based on
the obtained reference value. The total amount of data, which constitute
the histogram, is a fixed value.
A'=A-.alpha.A+.alpha.B
where A': a corrected frequency (number of pixels) corresponding to each
density of the present line, A: a frequency corresponding to each density
calculated up to the previous line, B: a frequency corresponding to each
density of the present line, and .alpha.: weighting coefficient.
The frequency value accumulated in each line is multiplied by the weighting
factor .alpha.. In other words, the weighting coefficient .alpha. shows a
contribution ratio to the histogram. The value of .alpha. is set in
accordance with the number of lines. Thus, the value is selected from 14
values (1/power of 2), i.e., 1, 1/2, 1/4, 1/8, 1/16, 1/32, . . . 1/2048,
1/4096, 1/8192 (=1/2.sup.13).
FIG. 7 is a flow chart showing an operation of the histogram preparation
circuit 80. The histogram preparation circuit 80 prepares a histogram from
data corresponding to the first scan line (ST1). The correction reference
value calculation section 81 calculates the reference value for the range
correction from the histogram prepared by the histogram preparation
circuit 80 (ST2). The histogram preparation circuit 80 calculates
(A')=A-.alpha.A for each frequency value of densities of the histogram
during the time between a line reading and next line reading, that is,
when no pixel density is inputted (ST3). The histogram preparation circuit
80 calculates A'=(A')+.alpha.B for each input pixel while reading one line
(ST4). In this way, the histogram preparation circuit 80 generates a
corrected frequency value of the present line, A'=A-.alpha.A+.alpha.B.
From the generated histogram, the reference value for a range correction
is calculated by the correction reference value calculation section 81
(ST5).
In the preparation of the histogram, two modes, that is, a mode 0 and a
mode 1 are set, and either one of modes is selected as required.
Mode 0: a weighting factor varied addition mode depending on the number of
scanning lines; and
Mode 1: a weighting factor fixed addition mode against the input pixel.
In mode 0, the value of coefficient .alpha. is varied in accordance with
the number of counts of the main scanning lines to prepare the histogram.
In mode 1, the coefficient is fixed, and the histogram is prepared
regardless of the count value of the main scanning line.
FIG. 8 is a block diagram showing the specific structure of the histogram
preparation circuit 80. Pixel density signals IDAT 4 to IDAT 7 are input
to one terminal of a switch 61 from the scanner section 1, and output data
signals CDT00 to CDT03 are input to the other terminal from a counter 63.
The switch 62 selects either one of input signals in accordance with a
selection signal from the timing signal generating section 83, and outputs
the selected signals SLDT0 to SLDT3 to a selector 66 and a clock
generating section 64. In this case, the pixel density signals IDAT4 to
IDAT7 correspond to upper 4 bits of the pixel density, and the pixel
density signals IDAT0 to IDAT3 are ignored for the above-mentioned reason.
The timing signal CTL0 sent from the timing signal generating section 83
is in a high level when the pixel density signal is not read, and the
switch 62 selects a signal from the counter 63 to be output.
The counter 63 supplies a necessary value to the clock generating section
64 and the selector 66 in calculating (A')=A-.alpha.A. When the
above-mentioned pixel density signal is not read, the counter 63 generates
a four-bit count value such that sixteen outputs of the clock generation
section 64 are selected and generated in order. A counter clock signal
CTICK is inputted from the timing signal generating section 83, and the
counter 63 is cleared by the counter clear signal CTICL from the timing
signal generating section 83. The counter clear signal CTICL is in a low
level when the pixel density signal is read, and the counter 63 is
cleared.
The clock signal generating section 64 selects one of sixteen outputs FCK0
to F at a period of an input clock signal MCK to be output in accordance
with the selection input signals SLDT0 to SLDT3. FIG. 9 shows the
relationship between I/O signals of the clock signal generating section
64.
Histogram registers (flip-flop) 65.sub.1 to 65.sub.F latch a corrected
frequency (WDAT) against each pixel density to be output when the input
clock signals FCK0 to F rise. The input signals WDAT is the
above-mentioned A'-.alpha.A or (A')+.alpha.B. Corrected frequency signals
H0 to HF sent from the histogram registers 65.sub.1 to 65.sub.F are also
output to the correction reference value calculation section 81.
The selector 66 inputs the frequency (the number of pixels) corresponding
to each density of 16 divisions from the histogram registers 65.sub.1 to
65.sub.F, and selects one data from 16 data H0 to HF (each bus width is 26
bits) to output a signal HSDT.
As shown in a timing chart of FIG. 10, a sub-scanning line number counter
76 outputs a line synch signal HDEN from the timing signal generating
section 83, and outputs count value signals FDAT00 FDAT12 to a clock
generating section 75. Then, the counter 76 is cleared by a clear signal
CRST sent from the main CPU 11 every time when one page of the document is
scanned.
The clock generating section 75 inputs output signals FDAT0 to FDAT12 sent
from the sub-scanning line number counter 76, and a pixel synch clock
signal GCK sent from the scanner section 1, and outputs a signal HCK to a
counter 74 and an addition value generating section 71. When the value of
the signal FDAT is one of 1, 3, 7, F, 1 F, 3 F, 1 FF, 3 FF, 7 FF, FFF, and
1 FFF, the clock generating section 75 outputs one clock of the input
pixel synch clock signal. The clock generating section 75 comprises an AND
circuit. When all line number signals FDAT are "1", that is, FDAT=1, 3
(11), 7 (111), F (1111), . . . , the clock generating section 75 outputs
one clock.
The counter 74 inputs the clock signal HCK from the clock generating
section 75, and outputs count value signals CDT20 to CDT23 to a selector
68 when the mode is set to 0. The counter 74 is also cleared by the clear
signal CRST sent from the main CPU 11 every time when one page of the
document is scanned. The count values CDT20 to CDT23 are values for
selecting .alpha. as shown in FIG. 6.
A fixed coefficient value register 78 outputs a fixed coefficient value
when the mode is set to 1. A switch 79 is changed in accordance with a
mode signal SL1 sent from the CPU 11. The switch 79 is set to the counter
74 when the mode is set to 0, and to the register 78 when the mode is set
to 1.
A subtraction value generating section 67 outputs ".alpha.A" in calculating
(A')=A-.alpha.A. The subtraction value generating section 67 inputs the
output signal HSDT from the selector 66, and generates a value, which is
obtained by dividing the signal HSDT by a power of 2 (the signal HSDT is
shifted).
The selector 68 determines ".alpha.A" of the calculation (A')=A-.alpha.A,
which is performed between the respective lines, that is, when the pixel
signal is not read, in accordance with input signals SSL0 to SSL3. In
other words, the selector 68 outputs (value of signal HSDT)/2.sup.2 when
the value of the input signal SSL0 is SSL3 is "1", and (value of signal
HSDT/2.sup.13 when the input value is C.
A subtraction section 70 performs a subtraction (A')=A-.alpha.A. The
subtraction section 70 inputs the density signal HSDT (A of the above
equation) from the selector 66 and a subtraction signal SDT (.alpha.A of
the above equation) from the selector 68, and outputs a signal YDAT as a
result of the subtraction.
The addition value generating section 71 (shift register) generates
".alpha.B" in calculating A'=(A')+.alpha.B. The addition value generating
section 71 inputs the clock signal HCK from the clock generating section
75, and outputs a signal XDAT to an adding section 69. The addition value
generating section 71 is also cleared by the clear signal CRST sent from
the main CPU 11 every time when one page of the document is scanned. FIG.
11 shows an example of the output of the addition value generating section
71. At the time of inputting the clear signal CRST, an initial value
output is 2000 H. Thereafter, every time when the clock signal HCK enters
from the clock generating section 75, 1/2 of the present value 2000 H is
1000 H, and 1/2 of the present value 1000 H is 800 H. FIG. 12 shows a
variation of each signal corresponding to the variation of the signal
FDAT.
The adding section 69 carries out addition A'=(A')+.alpha.B. The adding
section 69 inputs the frequency signal HSDT from the selector 66, and the
signal XDAT of addition data from the addition value generating section
71, and outputs a signal ZDAT as a result of the addition. FIG. 13 shows
an example of the addition of the signal ZDAT.
A switch 77 changes the calculations of (A')=A-.alpha.A and
A'=(A')+.alpha.B. The addition result signal ZDAT is input to one terminal
of the switch 77 from the adding section 69, and the subtraction result
signal YDAT is input to the other terminal from the subtraction section
70. One of inputs is selected in accordance with a selection signal CTL1,
and a selection result signal WDAT is output to the histogram registers
65.sub.1 to 65.sub.F.
The preparation of the histogram having the structure shown in FIG. 8 will
be explained with reference to timing charts of FIGS. 14 and 15.
FIG. 14 is a timing chart showing a state when A'=(A')+.alpha.B is
calculated every input pixel during one line reading. The signal MCK is a
main clock and synthesized with the pixel signal. A signal VDEN is a page
synch signal, and a signal HDEN is a line synch signal. The pixel density
signals IDAT4 to IDAT7 sent from the scanner section 1 are upper four bits
of the pixel density, and input to the switch 62. A sub-scanning effective
signal CTL0 is enable (low level) in this case. The switch 62 sends inputs
IDAT4 to IDAT7 to the selector 66 and the clock generating section 64.
The selector 66 selects the output (frequency) of the histogram registers
65.sub.1 to 65.sub.F in accordance with the pixel signals IDAT.sub.4 to
IDAT.sub.7, that is, the value of the selection input signal, and outputs
the selected frequency signal HSDT. A weighting factor (XDTA) is added to
the signal HSDT in accordance with the number of liens by the adder
section 69. Since the switch 77 is set to the adding section 69 by the
input signal CTL1 in this case, the addition result signal ZDAT is
returned to the histogram registers 65.sub.1 to 65.sub.F.
The clock generating section 64 outputs the clock signals FCK0 to FCKF in
accordance with the pixel signals IDAT4 to IDAT7. Each of the histogram
registers 65.sub.1 to 65.sub.F latches, i.e., stores the value of the
output signal WDAT of the switch 77 when each of the clock signals FCK0 to
FCKF rises. The above-mentioned processing is provided every pixel of one
line, so that the histogram of one line is generated, and the reference
value for adjusting the pixel density is calculated. The reference value
is used in the processing of the next line.
During the time, which is from one line reading to a next line reading,
that is, when the pixel density signal is not input, the equation,
(A')=A-.alpha.A, is calculated to obtain the frequency of each density of
the histogram.
FIG. 15 is a timing chart showing a state of the subtraction processing.
The switch 62 is changed to the counter 63 by the selection signal CTL0,
and the switch 77 is changed to the subtracter 70 by the selection signal
CTL1. The selector 68 subtracts each histogram value based on the
coefficient (in mode 0), which is determined by the number of sub-scanning
counters, or the fixed coefficient (in mode 1). After the subtraction is
ended, an operation is moved to the operation of the histogram preparation
of the next line.
By repeating the above-mentioned operations, in the case where the mode is
set to 0, the histogram in which total amount of data is constant is
prepared every time when each main scanning line is read. In the case
where the mode is set to 1 and the weighting factor is fixed, a histogram,
which can deal with the sharp change of the density of the document image,
can be obtained.
The following will explain the image data processing based on the histogram
of the present invention with reference to a flow chart of FIG. 16. In
this embodiment, it is discriminated whether or not input image data is
document image data, and the histogram is prepared by use of only document
image data.
First of all, image data read by the scanner section 1 is scanned to be
sampled by the histogram preparation circuit 80 of the image forming
processing section 44 (ST10). Then, a histogram of a first scanning line
is prepared (ST11). The histogram clear discriminating section 88
discriminates the white peak and the black peak of the histogram prepared
by the histogram preparation circuit 80 (ST12). The correction reference
value calculation section 81 determines a reference value for a density
adjustment (ST13).
FIGS. 17A to 17C are views each showing an example of a histogram by
scanning and sampling one line. FIG. 17A shows the histogram when the
document cover 109 is opened and the scanner section 1 reads a
non-document area, that is, an area other than the document. In this case,
the peak of the histogram is one-sided to the black side (high density).
FIG. 17B shows the histogram when the document cover 109 is closed and the
scanner section 1 reads a white document cover. In this case, the peak of
the histogram is one-sided to the white side (low density). FIG. 17C shows
the histogram when the normal image (e.g., newspaper) reading is carried
out.
As shown in FIGS. 17A to 17C, when the document reading and the reading of
the non-document area are performed, the histogram is characteristically
one-sided in their distributions. According to the present invention, the
histogram whose distribution is extremely one-sided is ignored, and
histograms obtained by use of only document image data are accumulated.
Thereby, a suitable correction reference value is calculated, and the
automatic density adjustment is performed.
In the histogram clear discriminating section 88, a white side threshold
value Wth and a black side threshold Bth are set in advance. The peak
(white peak) of the histogram of the white side in the histogram
scan-sampled on one line is compared with the threshold value Wth, and the
peak (black peak) of the histogram of the black side is compared with the
threshold value Bth (ST14).
In a case of the document like newspaper shown in FIG. 4A, one curve of the
histogram can be formed at a relatively thin density portion corresponding
to the background portion, and one curve can be formed at a relatively
thick density portion corresponding to the black character portion.
Therefore, the threshold value Wth of the white side and the threshold
value Bth of the black side are set in consideration of the distribution
state of the histogram.
For example, as shown in FIG. 17A, in a case where the position of the
black peak is placed at a black side rather than the threshold value Bth,
it can be discriminated that this is not the document reading. As shown in
FIG. 17B, in a case where the position of the white peak is placed at a
white side rather than the threshold value Wth, it can be discriminated
that this is not the document reading. In a case of FIG. 17C, since the
position of the white peak is placed at a black side rather than the
threshold value Wth, and the position of the black peak is placed at a
white side rather than the threshold value Bth, it can be discriminated
that this is the document reading.
In other words, in the case where the histogram clear discriminating
section 88 discriminates that the position of the white peak is placed at
the black side rather than the threshold value Wth and the position of the
black peak is placed at the white side rather than the threshold value
Bth, the histogram preparation circuit 80 executes the normal accumulative
histogram preparation processing (ST15).
Also, in the histogram clear discriminating section 88, if the detected
position of the black peak is placed at the black side rather than the
threshold value Bth or the position of the white peak is placed at the
white side rather than the threshold value Wth, the operation goes to the
following step.
The histogram clear discriminating section 88 discriminates whether or not
a sum of the frequencies of n blocks which centrally include one of higher
frequencies of the white peak and the black peak, is a predetermined value
Z or more (step ST 16). The reason why the density of the peak frequency
of only one of higher frequencies of the white peak or the black peak is
counted is as follows.
More specifically, if both white peak frequency and the black peak
frequency are counted, the normal character document may be included. In
other words, the above method is used to omit data of the area, which can
be considered to be unnecessary.
If the sum of the frequencies of n blocks is less than the predetermined
value Z, the histogram preparation circuit 80 prepares the accumulative
histogram by use of the histogram data. If the total amount of the
frequencies of n blocks is the predetermined value Z or more, each
frequency stored in the histogram preparation circuit 80 (651 to 65F) is
cleared by the histogram clear discriminating section 88 (step ST17).
Then, a preparation of a new histogram is executed by use of read data of
a next line. In other words, in the cases of FIGS. 17A and 17B, the the
histogram clear discriminating section 88 discriminates that the document
is not correctly placed on the document glass 92 of the document plate 117
and that the scanner section 1 scans and samples the space between the
document and the document scale 91. Therefore, the respective frequencies,
which are stored in the histogram preparation circuit 80 (651 to 65 F) as
a result of scanning the above space by the scanner section 1, are all
cleared.
It is noted that the threshold values Wth and Bth of the histogram clear
discriminating section 88 are variable by the setting of the main CPU 11.
Also, the predetermined value Z for the sum of the frequencies of n blocks
of the histogram clear discriminating section 88 is variable by the main
CPU 11. Moreover, by the result of the various experiments, a routine for
clearing the histogram is carried out when the sum is 92 to 93% or more of
the total number of pixels of one line.
As mentioned above, according to the present invention, since the portion
other than the document, that is, black data and white data of the
non-document portion are not used as histogram data, a suitable histogram
can be prepared. Therefore, the automatic density adjustment can be
executed based on the suitable correction reference value.
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
invention and, together with the general description given above and the
detailed description of the preferred embodiments given below, serve to
explain the principles of the invention.
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